[lldb/Plugins] Use external functions to (de)initialize plugins

This is a step towards making the initialize and terminate calls be
generated by CMake, which in turn is towards making it possible to
disable plugins at configuration time.

Differential revision: https://reviews.llvm.org/D74245

lldb/source/Plugins/ObjectFile/wasm/ObjectFileWasm.cpp

@@ -28,6 +28,8 @@ using namespace lldb;
 using namespace lldb_private;
 using namespace lldb_private::wasm;
 
+LLDB_PLUGIN(ObjectFileWasm);
+
 static const uint32_t kWasmHeaderSize =
     sizeof(llvm::wasm::WasmMagic) + sizeof(llvm::wasm::WasmVersion);